Planar inductors may be guided, in an advancing operation, over a surface to be hardened of a metallic workpiece. The conductor loop is located here in a plane which is parallel to the surface to be hardened, wherein a predetermined spacing gap remains between the conductor loop as the induction coil and the surface. In order to permit as efficient heating up by means of induction as possible, a small spacing gap within the millimeter range or submillimeter range is generally set. On the other hand, however, direct contact between the induction coil and the workpiece to be hardened also has to be avoided in order to avoid local overheating and adhesion of material due to melting.
The planar inductor can be used in particular in the case of a large rolling bearing for hardening a running surface. Corresponding methods are known from DE 10 2005 006 701 B3 and DE 10 2008 033 735 A1.
The inductors which are merely illustrated schematically in the documents mentioned have a conductor loop with two parallel limbs The limbs here are oriented perpendicularly to the direction of movement of the inductor during the progressive surface layer hardening and can be arranged on a carrier composed of a flux concentration material. With the inductor, the surface arranged therebelow is heated to a temperature which causes a structural conversion of the surface layer, with the hot surface subsequently being quenched in the advancing operation, for which purpose, for example, a cooling liquid is applied with a sprinkler.
In order, during the surface layer hardening in the advancing operation, to be able to maintain a predetermined distance between the planar, customarily flat inductor and the surface to be hardened, the inductor is customarily arranged so as to be adjustable by means of an adjustment mechanism or in a floating manner, wherein measurement of the distance takes place with a measuring roller arranged next to the inductor. The use of a measuring roller has proven successful in practice. If, however, as per a method according to DE 10 2005 006 701 B1, two inductors are moved in an opposed direction on an annular surface, said inductors have to be brought up to each other as close as possible at the beginning and at the end of the surface layer hardening, and therefore the use of a laterally arranged roller is then not readily possible. Furthermore, the roller does not serve for the direct force resistance, but merely for measuring the distance, with the gap size then being tracked by a suitable adjustment. The known system cannot keep the gap constant under all circumstances at the beginning of the hardening operation because considerable attraction forces can result precisely by switching on the currents at the beginning, said attraction forces drawing the inductor onto the surface to be hardened, in which case damage is then not ruled out.
In order to avoid such contact during the hardening of large rolling bearings, it is known to guide a plastics strip made from polytetrafluoroethylene between the inductor and the surface to be hardened immediately on switching on the currents. The plastics strip then serves as a spacer and avoids the inductor striking directly against the metallic surface to be hardened. With the reaching of a constant alternating current, the attraction forces then decrease, and therefore the plastics strip can be removed by a user. If the plastics strip remains too long between the inductor and the surface to be hardened, there is the risk of the plastics strip melting or disintegrating, as a result of which soiling may occur.